1. Field of the Invention
This invention relates to the optical system of an albada finder used in a camera or the like.
2. Related Background Art
An inverted Galilean type albada finder is often adopted as the finder of a compact camera or the like. The inverted Galilean type albada finder according to the prior art is constituted by an objective of negative refractive power and an eyepiece of positive refractive power disposed on that side of the objective which is adjacent to the eye side, and a field frame is formed on the lens surface in the eyepiece by evaporation of a metal. In order to reflect the light from the field frame and enable the image of the field frame to be observed substantially with the same visibility as the object image, a half-mirror or a reflecting surface having only its marginal portion formed into a reflecting portion (hereinafter referred to as the albada reflecting surface) is provided in the objective. The optical system leading from the field frame via the albada reflecting surface to the observer side through a lens provided on the opposite side of the object with respect to the field frame will hereinafter be referred to as the albada optical system. Also, it is to be understood that the field frame includes a plurality of indexes indicating the area exposed to a film. A construction has heretofore been conceived in which display of the measured distance, display of the warning of improper exposure, display of the preparation for strobo light emission, etc. can be observed near the image of the field frame by the use of such albada optical system. For the purposes of said displays, it is necessary to dispose a prism and pointers near the field frame. To effectively accomplish the disposition of such various display members, it is necessary to keep the combined focal length of the albada optical system long so that the display prism and the display pointers do not cover the field of view of the finder to make a part of the field of view difficult to see.
Where the focal length of the albada optical system is short, as compared with a case where the focal length of the albada optical system is long, it is necessary to make the size of the field frame small, and further, the deviation of the optical path near the field frame between the light beam travelling from the field frame toward the albada reflecting surface and the light beam from the field frame reflected by the albada reflecting surface becomes great. Accordingly, to enable the display members to be observed as if they were near the field frame and to prevent the display members from making an object to be photographed difficult to observe, it becomes necessary to make the combined focal length of the albada optical system long.
Also, if the focal length is short, the field frame is therefore made small, the eccentricity of the lens of the finder optical system is greatly affected.
In recent years, to make the finder optical system compact and make the finder capable of being zoomed or capable of changing the magnification, the construction has become complicated and it has become difficult to provide sufficient space of the albada optical system in the direction of the optic axis and along therewith, it has become difficult to effect the various types of information display as described above.